We have applied two-color resonant two-photon mass analyzed threshold ionization spectroscopy to investigate some ionic properties of N-alkylanilines. The respective adiabatic ionization energies of N-methylaniline (NMA), N-ethylaniline (NEA), and N,N-dimethylaniline (DMA) are determined to be 59 822, 59 204, and 58 018 cm−1 with an uncertainty of about 5 cm−1. This indicates that the longer alkyl chain gives rise to a larger redshift in the IE due to a stronger interaction between the alkyl group and the nitrogen atom in the ionic state. Because the alkyl group gives rise to an increase in the electron density around the nitrogen atom of the neutral species, the IE of DMA is lower than that of NMA. In addition, the N-alkyl substitution also influences the frequency of the internal motion of the cations. However, the frequency variation is dependent upon the vibrational pattern and the extent of the coupling between the N-alkyl group and ring vibrations.
We have applied mass analyzed threshold ionization (MATI) spectroscopy to record the vibrational spectra of the indoline cation by ionizing through several vibrational levels in the S1 state. The adiabatic ionization energy is determined to be 59 269±5 cm−1. The present experimental results show that only one stable configuration is involved in the photo-excitation processes, as predicted by ab initio calculations. Comparing the electronic transition and ionization energies of indoline with those of aniline and N-ethylaniline suggests that the cyclization of the five-membered ring leads to a redshift in both quantities. When the intermediate states involve large amplitude vibrations, the intense bands that appeared in the MATI spectra correspond to similar vibrational patterns, indicating that the structure of indoline in the D0 state resembles that in the S1 state. The puckering of the five-membered ring, the ring flopping (butterfly), the out-of-plane ring twisting, and the N-inversion of indoline in the D0 state are observed with frequencies of 92, 187, 235, and 706 cm−1, respectively. These spectral bands may be regarded as the characteristic features of the indoline cation for molecular identification.
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